HRSV was discovered in 1956 and is found worldwide. It causes upper and lower respiratory tract disease particularly in infants and young children. About 30 percent of hospitalized young children with acute respiratory disease have respiratory syncytial virus infection. In older children and adults the disease is milder. In infants this severe illness often requires hospitalization.
Infections with respiratory syncytial virus are referable to all segments of the respiratory tract, are usually associated with fever, cough, runny nose, and fatigue, and are diagnosed clinically as bronchitis, bronchiolitis, pneumonia, croup, or viral infection. In older children and adults the virus is generally limited to replication in the upper respiratory tract. Infants may be more severely involved when the virus extends into the lungs. Lung damage can be permanent.
Primary infection with respiratory syncytial virus occurs early in life, usually before 4 years of age. Among children, illness caused by this virus tends to occur at least once each year in rather sharply defined outbreaks of several months duration. Epidemics are sharply circumscribed, generally for 3 to 5 months. In family studies, children in early school years frequently introduce the virus into the home, infecting younger members of the family more severely than other family members. The clinical consequence of infection is most severe on first experience and becomes milder in older individuals who are immunologically experienced.
Secondary effects of respiratory syncytial virus can range from inapparent infection to severe pneumonia and death. Inflammation of the respiratory tract is responsible for most symptoms. Complete recovery in most cases occurs in one to three weeks with the production of antibody which appears to persist throughout life. In the United States about 30 percent of 1-year-old infants and 95 percent of 5-year-old children have circulating respiratory syncytial virus antibody. Reinfections in older infants, children, and adults with antibody are mostly mild upper respiratory illnesses in the form of colds.
Although low yields of virus in cell culture have hindered HRSV research, the virus has been well studied. HRSV is a paramyxovirus containing a single negative strand of RNA which is transcribed into 10 predominantly monocistronic messengers. The messengers have been isolated and translated in vitro. The products have been characterized by gel electrophoresis, peptide mapping and immuno-precipitation as being similar to structural proteins isolated from virions. The structural proteins include a major nucleocapsid protein (N; MW ca. 42,000), a nucleocapsid phosphoprotein (P; MW ca. 34,000), a large nucleocapsid protein (L; MW ca. 200,000), an envelope matrix protein (M; MW ca. 26,000), a matrix glycoprotein (ca. 22,000) and two envelope glycoproteins, the fusion glycoprotein (F; MW ca. 68,000 to 70,000) and a second, methionine poor glycoprotein (G; MW ca. 84,000 to 90,000). In addition, a virally encoded protein of about 9,500 daltons and other small proteins are known to be present in infected cells, Collins, et al., Identification of a tenth mRNA of HRSV and assignment of polypeptides to the 10 viral genes, J. of Virol. 49:572-578 (1984) and references cited therein. Additional work describing the molecular biology of HSRV includes: (1) Collins, et al., Nucleotide Sequence of the gene encoding the fusion (F) glycoprotein of human respiratory syncytial virus, Proc. Natl. Acad. Sci., USA, 81:7683-7687 (December 1984) disclosing the gene sequence for the F glycoprotein: (2) Collins, et al., The 1A Protein Gene of Human Respiratory Syncytial Virus: Nucleotide Sequence of the mRNA and a Related Polycistronic Transcript, Virology, 141:283-291 (1985) disclosing the gene sequence for the 1A protein; (3) Collins, et al., The Envelope-Associated 22K Protein of Human Respiratory Syncytial Virus: Nucleotide Sequence of the mRNA and a Related Polytranscript, J. of Virol., 54(No.1):65-71 (April 1985) disclosing the gene sequence for the 22K protein; (4) Wertz, et al., Nucleotide sequence of the G protein gene of human respiratory syncytial virus reveals an unusual type of viral membrane protein, Proc. Natl. Acad. Sci., USA, 82:4075-4079 (June 1985) disclosing the gene sequence for the G glycoprotein; and (5) Collins, et al., Correct Sequence for the Major Nucleocapsid Protein mRNA of Respiratory Syncytial Virus, Virology, 146:69-77 (1985) disclosing the gene sequence for the .infin. protein.
The F and G glycoproteins of HRSV have similar counterparts in the other paramyxoviruses. Like HRSV, other paramyxoviruses have an F glycoprotein which is associated with fusion of cell membranes, P. W. Choppin and A. Scheid, Rev. Infect. Dis. 2:40-61, (1980); Merz, et al., J. Exp. Med. 151:275-288, (1980). The active paramyxovirus F protein consists of two disulfide-linked subunits, F.sub.1 and F.sub.2, which are generated from an inactive precursor (F.sub.0) by a specific internal cleavage by cellular proteases, Scheid and Choppin, Virol. 80:54-66 (1977). The second major glycoprotein for most paramyxoviruses is termed the HN protein, and is associated with the hemagglutinin and neuraminidase activities of these viruses. Although the HRSV G protein does not have the above enzymatic activities, both the G and HN glycoproteins are associated with attachment of virus. Also, these glycoproteins are structurally similar in that they have an unusual hydrophobic signal/anchor region at their amino-terminus, Wertz, et al., PNAS 82:4075-4079 (1985); Elango, et al., J. Virol. 57:481-489 (1986).
There are no available effective vaccines to combat HRSV. Multiple attempts have been made to obtain an effective vaccine against HRSV. Friedewald, et al., Journal of the American Medical Association, 204:690-694 (May 20, 1968), describe the propagation of respiratory syncytial virus in bovine embryonic kidney tissue culture. Virus grown at 34.degree. C. or 28.degree. C. did not decrease in infectivity or virulence. HRSV grown at 26.degree. C., while associated with a decrease in infectivity for adults, could not be considered for use in prevention of infection in adults since the virus had limited infectivity and was poorly immunogenic.
Kim, et al., Pediatrics, 48:745-755 (November 1971) disclose that inactivated respiratory syncytial virus vaccine prepared from virus grown at 26.degree. C. stimulated the development of high levels of serum antibody in infants and children from 6 months to 13 years in age but did not prevent infection.
McIntosh, et al., Pediatric Research, 8:689-969 (1974) discuss two experimental live respiratory syncytial virus vaccines, one prepared from virus grown at 26.degree. C. and the other, prepared from a temperature sensitive mutant which grew well at 32.degree. C. and not at all at 37.degree. C. or higher. The first vaccine was unsatisfactory as it did not protect against infection when the interval between vaccination and challenge was greater than 4 months. The second vaccine was also unsatisfactory in that it apparently lost its temperature sensitivity in some vaccinees.
Craighead, Journal of Infectious Diseases, 131:749-753 (June 1975) discusses tests conducted in 1966 wherein several groups of investigators tested in infants and young children a formaldehyde-treated, alum-precipitated virus grown in tissue culture. Upon subsequent exposure to wild virus the vaccine recipients exhibited an accentuated pattern of respiratory tract disease. Craighead concludes that immunization with formaldehyde treated virus enhanced the severity of the disease.
Wright, et al., Journal of Pediatrics, 88:931-936 (June 1976) describe the evaluation in infants of a temperature sensitive live attenuated respiratory syncytial vaccine. While this vaccine when administered at a dosage level sufficiently high to infect all seronegative infants caused mild upper respiratory illness, lowering the dose did not achieve an acceptable level of infectivity. The virus was also genetically unstable as there was evidence of loss of temperature sensitivity in one vaccinee. There was no evidence for potentiation of natural illness with this vaccine and reinfection occurred among vaccinees.
U.S. Pat. Nos. 4,122,167 and 4,145,252 describe a method for attenuating virions by serial passage through human diploid lung fibroblasts and U.S. Pat. No. 4,517,304 discloses a method for producing immunogenically active HRSV proteins upon the cell membranes of susceptible cells grown in culture. These cells are then injected into a host to elicit an immune response.